A magnetic disk drive includes circular data tracks on a rotating magnetic disk and read and write heads that may form a merged head attached to a slider on a positioning arm. During a read or write operation, the merged head is suspended over the magnetic disk on an air bearing surface (ABS). The sensor in a read head is a critical component since it is used to detect magnetic field signals by a resistance change. The resistance change is produced by a giant magnetoresistance (GMR) effect which is based on a configuration in which two ferromagnetic layers are separated by a non-magnetic conductive layer in the sensor stack. One of the ferromagnetic layers is a pinned layer in which the magnetization direction is fixed by exchange coupling with an adjacent anti-ferromagnetic (AFM) or pinning layer. The second ferromagnetic layer is a free layer in which the magnetization vector can rotate in response to external magnetic fields. The rotation of magnetization in the free layer relative to the fixed layer magnetization generates a resistance change that is detected as a voltage change when a sense current is passed through the structure. In a CPP configuration, a sense current is passed through the sensor in a direction perpendicular to the layers in the stack. Alternatively, there is a current-in-plane (CIP) configuration where the sense current passes through the sensor in a direction parallel to the planes of the layers in the sensor stack. The CPP GMR head design has been previously disclosed in U.S. Pat. Nos. 5,627,704 and 5,668,688.
A CPP GMR head is considered to be a promising candidate for >200 Gb/in2 recording density. The CPP configuration is more desirable for ultra-high density applications because a stronger output signal is achieved as the sensor size decreases, and the magnetoresistive (MR) ratio is higher than for a CIP configuration. Both CPP and CIP GMR heads may take advantage of a CoFe/NiFe composite free layer for improved performance. In the CPP GMR head structure, a bottom spin valve film stack is generally employed for biasing reasons as opposed to a top spin valve where the free layer is below a copper spacer and the pinned layer is above the copper spacer.
An important characteristic of a GMR head is the MR ratio which is dR/R where dR is the change in resistance of the spin valve sensor and R is the resistance of the spin valve sensor before the change. A higher MR ratio is desired for improved sensitivity in the device and this result is achieved when electrons in the sense current spend more time within the magnetically active layers of the sensor. Interfacial scattering which is the specular reflection of electrons at the interfaces between layers in the sensor stack can improve the MR ratio and increase sensitivity. Unfortunately, the MR ratio is often very low (<5%) in many CPP-GMR spin valve structures involving metal spacers. A MR ratio of ≧10% and an RA of <0.5 ohm-um2 are desirable for advanced applications.
A synthetic anti-parallel (SyAP) pinned layer configuration represented by AP2/coupling/AP1 is preferred over a single pinned layer because a smaller net magnetic moment is possible for a SyAP layer and that means greater exchange coupling between the AFM layer and adjacent AP2 layer. It is also known in the art that a laminated AP1 layer made of alternating CoFe and thin Cu layers can improve the MR ratio in CPP-GMR heads as described by M. Takagishi (Toshiba) in a TMRC2001 presentation. An example of a CPP-GMR bottom spin valve structure is represented by a seed/AFM/pinned/spacer/free/cap configuration where seed is a seed layer, the pinned layer has a [AP2/coupling/AP1] SyAP configuration in which Ru may be the coupling layer and [CoFe/Cu/CoFe] is the laminated AP1 layer, and the free layer is a CoFe/NiFe composite. AP1 and AP2 thickness is typically in the range of 20 to 50 Angstroms and the free layer thickness is from 30 to 60 Angstroms. For read head applications, the free layer preferably has a small coercivity (Hc) of less than 10 Oersted (Oe) and a low magnetostriction on the order of 10−8 to low 10−6 values to reduce stress induced anisotropy.
Another important consideration for CPP-GMR read heads is electromigration (EM) performance. For CPP spin valve structures having an AP2/coupling/AP1 pinned layer configuration, Fe rich CoFe alloys such as Fe50Co50 or Fe70Co30 in the AP1 layer are known to be effective in enhancing the MR ratio. However, Fe rich CoFe alloys may cause EM performance degradation. Therefore, it is important to control EM performance through proper choice of the spacer layer and AP2 layer.
CPP GMR head performance can be improved by employing a confining current path (CCP) in a copper spacer through means of segregating metal path and oxide formation as described in U.S. Pat. No. 5,715,121. A CCP Cu spacer structure can be represented as [Cu/CCP layer/Cu] where the CCP layer may be formed by co-depositing Al2O3 and Cu, for example. Another example of a confining path spacer is described in U.S. Patent Application 2006/0209472 where a columnar metal path penetrates an insulating portion vertically and the insulating portion comprises over 50% of the spacer. Although the MR ratio is relatively large for a CCP-CPP scheme, the uniformity and EM are two major obstacles that require development before the CCP technology can be incorporated in a product.
In U.S. Pat. No. 7,116,529, a non-magnetic spacer is disclosed that has a Cu/AlCuO/Cu stacked structure to provide a current confinement effect. The oxide portion is between 5 and 50 Angstroms thick.
U.S. Pat. No. 6,876,523 describes a spacer made of a noble metal such as Pt, Pd, Rh, Ru, Ir, Au, or Ag to achieve an improved MR ratio compared to a Cu spacer.
In U.S. Pat. No. 6,917,088, a Au spacer is replaced by an oxide semiconductor spacer having d-electrons at the Fermi surface. Two examples are SrTiO as a perovskite type oxide and TiO2 as a rutile type oxide.
U.S. Patent Application 2006/0060901 teaches a spacer layer made of a low resistance material (Cu, Au, Ag, Al) or a high resistance material which is an insulator made of an oxide, nitride, or fluoride of Al, Ti, Ta, Co, Ni, Si, Mg, or Fe.